Author: Omid David Tabibi
Date: 03:32:25 10/12/03
Recently I conducted some extensive experiments with two versions of Falcon, one with checks in quiescence and one without. Falcon already has lots of extensions, but adding checks in quiescence resulted in a significant boost for tactical strength. I tested the following options: a) checks everywhere in quiescence b) checks only in the first ply of quiescence c) no checks in quiescence Option 'a' was ruled out after some testing, as it resulted in a total explosion of quiescence search. I tried controlling it in some ways, but still the overhead was considerably more than the benefit. It seems that The King and HIARCS are the only engines using this method. Option 'b' produces almost the same tactical strength as option 'a', with a considerably lower overhead. The most significant contribution of checks in the first ply of quiescence seems to be in conjuction with null-move pruning near leaf nodes: For example at depth = 3, using R = 2, the null-move search will be called with a depth of 0, i.e., direct call to quiescence search. Here the presence of checks in the first ply can return a checkmate value which will result in an extension to main search (mate threat extension). Only using checks in the first ply of quiescence, Falcon managed to solve almost all tactical positions of LCTII in less than 1 second, outperforming the normal version (no checks in quiescence). But adding checks in quiescence (although only at its first ply) significantly slowed down the engine (from average of 350kNPS to 150kNPS on my PIII/733MHz) and resulted in a worse branching factor. Next I conducted some self-play matches between the two versions, and also some matches versus other engines. The results of the matches were quite interesting. The version with checks in quiescence not only didn't outperform the normal version in actual games, but produced slightly inferior games in general. I especially conducted a few tens of matches for each version against Crafty. The normal version beat Crafty by something like 60%-40%. The version with checks in quiescence scored 50%. Whenever the game turned tactical it literally butchered Crafty, but on normal quiet positions Crafty once and again made a mincemeat of it by simply searching deeper, which resulted in a better positional play. So, it seems that adding checks in quiescence is great for solving tactical test suites, but not so for actual game play. The same goes for some of the aggressive extensions I tried; great for tactics, poor in games. I'd be interested to hear others' thoughts on this issue. I also considered using some form of static mate threat detection, independent of null-move search, but haven't found any interesting way to do so yet. Also, Falcon does not detect checkmates statically in eval(), but only when one side doesn't have any legal moves, i.e., it needs an additional ply to see the checkmate. But I don't think the latter is any important, since when the other side is checked, a check extension is already done, which will result in the detection of the checkmate.
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